"But you were unwilling to go up; you rebelled against the command of the Lord your God. You grumbled in your tents and said, 'The Lord hates us; so he brought us out of Egypt to deliver us into the hands of the Amorites to destroy us. Where can we go? Our brothers have made our hearts melt in fear. They say, "The people are stronger and taller than we are; the cities are large, with walls up to the sky. We even saw the Anakites there."'" NIV translation
Moses reminds the people that they had refused to trust in God and to follow His instructions. Even though Moses had made a mistake in sending the spies, they still had the opportunity to have faith in God and to go in and take possession of the land.
The same thing applies to us in our Christian walk. No matter what others (the spies) say, we can trust in what God has revealed to us. We have to choose to follow Him no matter what we see or think.
"Then I said to you, 'Do not be terrified; do not be afraid of them. The Lord your God, who is going before you, will fight for you, as he did for you in Egypt, before your very eyes, and in the wilderness. There you saw how the Lord your God carried you, as a father carries his son, all the way you went until you reached this place.'" NIV translation
Moses continues by reminding them that he had tried to tell them that God would take care of them. He reminded them of what God had done for them in the past and that is a picture of our testimony as a Christian. When we see others that are struggling, we can share what God has done for us in the past to build them up in their faith.
"In spite of this, you did not trust in the Lord your God, who went ahead of you on your journey, in fire by night and in a cloud by day, to search out places for you to camp and to show you the way you should go." NIV translation
Even with the past experiences, the people had refused to trust in the Lord. Moses reminded them that they had the pillar of fire and the cloud as a visual representation of the fact that God was with them but they still refused to trust in Him.